1. Field of the Invention
The invention relates to an apparatus and method for measuring heat-flow in three dimensions in the subsurface regime. More particularly, it relates to a triaxial thermopile array apparatus and method for its use wherein the apparatus includes an array of oriented heat flow transducers and, optionally, an array of thermistors which are buried in a location so as to provide for measurement of transducer voltages and thermistor resistances in order to determine heat flow in three dimensions.
2. Background of the Invention
Information about heat transfer in the subsurface regime is important for assessing potential sites for geothermal power plants. Subsurface heat transfer is also indicative of groundwater flows which must be studied for their contamination by and their possible effect on existing and proposed chemical and nuclear waste repositories.
Previous methods of measuring heat flow have depended upon temperature measurements taken at various levels in boreholes. From the temperature gradients thus obtained and Fourier's law for heat conduction, estimates of heat flow can be obtained. There are, however, severe complications associated with using temperature differences in a borehole to estimate heat flow. First, temperature differences in a borehole give estimates of heat flow along one axis only, i.e., along the borehole. Second, if groundwater flow is present, most of the heat may be transported by convection causing either measurable temperature differences to vanish and/or Fourier's law to be inapplicable. Third, if hydrologic flows are present, heat transfer in and around the borehole may not be indicative of heat transfer at some distance from the borehole in the porous regime.
U.S. Pat. No. 4,420,974 (to Lord) discloses an in-situ measurement system which involves using a series of rigid reinforcing bars having an open loop or "hairpin" configuration as temperature sensors. The device disclosed by Lord depends on the rather weak variation of electrical resistance with temperature in "rebar" to measure in-situ temperatures over large depth intervals (20 ft or more) in a borehole. The design and technique disclosed by Lord are not adequate or appropriate for measuring interborehole temperature differences of the minuscule magnitude associated with geothermal gradients. This lack of sensitivity means that typical geothermal horizontal temperature gradients cannot be reliably measured using the device. The measurement device requires current to be passed through the "rebar" sensors. Using the voltage-current information from the device disclosed by Lord, it is calculated that ohmic heating in the "rebar" will swamp much smaller geothermal heat flows. The rebar sensors in this device are in contact with grout, and electrical leakage into surrounding groundwater that is likely to penetrate the grouting can have a larger effect on measured voltages than the small temperature variations associated with geothermal heat flows.
U.S. Pat. No. 3,874,232 (to Hardison) discloses a device which uses thermistors or thermocouples clamped in a borehole to obtain temperature differences along the borehole length. This device uses the open borehole approach which is subject to interpretive errors. The measurement system disclosed by Hardison yields information about vertical heat flows only, since only vertical temperature gradients are measured using this system.
Thus, both the devices disclosed by the Lord '974 and Hardison '232 patents fail to overcome the aforementioned problems including failure to measure temperature differences along more than one axis and failure to take into account groundwater flows as well as hydrologic flows.
Heat flow sensors, such as heat flow transducers, have been employed for measuring near surface vertical heat flow in geothermal regions. Dunn et al, "Surface heat flow measurements at the Puhimau hot spot," Geophysics, Vol. 50, No. 7, July 1985, pp. 1108-1112. However, prior to the present invention, triaxial measurements of geothermal heat flow using heat flow transducers have not yet been attempted.